Traffic approaching the George Washington Bridge in Fort Lee, N.J., this month. The opening arguments started on Monday in the trial that stemmed from the closing of access lanes to the bridge in 2013.CreditCreditDrew Angerer/Getty Images

NEWARK — Gov. Chris Christie of New Jersey knew that three of his top officials were involved in a plan to shut down lanes leading to the George Washington Bridge as it was happening and that the closings were intended to punish a local mayor for declining to support him, federal prosecutors said on Monday.

The assertion was an unexpected and startling beginning to the trial of two former Christie administration officials charged with closing the lanes in 2013 and then covering it up. And it was a surprising claim because of the side of the courtroom it came from, as lawyers made opening statements.

Defense lawyers have long argued that Mr. Christie, a Republican, and his top advisers were well aware of the lane closings and that they directed the cover-up as they tried to protect the governor’s political aspirations — saying their clients were “thrown under the presidential bus,” as one lawyer argued on Monday.

But this was the first time a prosecutor had pointed a finger at Mr. Christie. And it directly contradicts the governor’s statements in the three years since the lanes were mysteriously closed, paralyzing the borough of Fort Lee, N.J.

Mr. Christie, a former top federal prosecutor in New Jersey, has consistently denied that he knew about the lane closings as they unfolded, and argued that the United States attorney’s office had “exonerated” him when it declined to indict him along with the defendants now standing trial.

The prosecutor, speaking for the United States attorney’s office, said that two of the alleged co-conspirators in the case, David Wildstein and Bill Baroni, had “bragged” to the governor about the lane closings at a memorial service for the Sept. 11 terrorist attacks, on the third day of the closings, and that they had been done to “mess” with Mayor Mark Sokolich of Fort Lee, a Democrat, because he had declined entreaties to endorse the governor’s re-election. Mr. Baroni and Bridget Anne Kelly, who were close allies of Mr. Christie, are the two defendants in the trial.

Mr. Wildstein and Mr. Baroni boasted to Mr. Christie that panicked phone calls from Mr. Sokolich, pleading that the lane closings were a “public safety emergency,” were deliberately being ignored, the prosecutor said.

The prosecutor, Vikas Khanna, quickly moved to quell any curiosity the jury might have about why Mr. Christie was not charged in the plot.

“The evidence may show that others could have, should have, perhaps known certain aspects of what was going on in Fort Lee,” he said. “Perhaps you will even wonder what happened to those people. But at the end of this case the only issue for you to decide is whether Bridget Kelly and Bill Baroni are guilty of the crimes with which they are charged beyond a reasonable doubt. That’s it.”

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Gov. Chris ChristieCreditMel Evans/Associated Press

Defense lawyers quickly seized on his comments in their own opening statements.

Michael Critchley, a lawyer for Ms. Kelly, a former deputy chief of staff to Mr. Christie, said: “We know who they’re talking about. They’re talking about Governor Christie. They’re talking about Kevin O’Dowd.” (Mr. O’Dowd was the governor’s chief of staff at the time of the closings.)

A spokesman for Mr. Christie, Brian Murray, responding to the prosecutor’s assertion, referred reporters on Monday to statements the governor made in 2014, in which he said he had not known about the plan to close the lanes. Mr. Murray declined to address whether the governor knew about the closings while they were happening.

The office of the United States attorney, Paul J. Fishman, declined to comment on why the governor was not charged, but Mr. Fishman has said in the past that merely knowing is not a federal crime.

Mr. Christie was intimately woven into the story both sides told in the packed courtroom on Monday.

Prosecutors and defense lawyers alike described an administration tightly controlled by the governor, one that worked hand-in-hand with his re-election campaign to trade favors for endorsements. The governor was trying to win a big and broad margin of victory, and to win over Democratic mayors like Mr. Sokolich, so he could make the case that he was the Republican best able to win the White House.

Mr. Khanna said Mr. Baroni covered up the lane closings because “this was something that was important to Trenton.”

He added, “Trenton: the governor’s office.”

Mr. Baroni’s lawyer, Michael Baldassare, later told the jury it would be hearing a lot about Trenton. “Trenton, Trenton, Trenton; Trenton is the governor,” he said. “Let’s make no mistake. Open a dictionary: Trenton, the governor.”

The details of the plot that Mr. Khanna laid out are largely familiar by now — that Ms. Kelly sent an email in August 2013 saying “Time for some traffic problems in Fort Lee” after confirming that the mayor of that borough would not endorse Mr. Christie. A month later, two of three access lanes to the George Washington Bridge were shut down. Mr. Baroni, then the highest-ranking official at the Port Authority of New York and New Jersey, which runs the bridge, studiously ignored the mayor as he pleaded by text, email and a handwritten letter for the agency to reopen the lanes.

Mr. Critchley, Ms. Kelly’s lawyer, said Mr. Christie and Gov. Andrew M. Cuomo of New York developed a strategy to cover up the lane closings as soon as they ended, and executed it through official statements over the next several weeks, lying that the closings had been a “traffic study.”

Mr. Critchley characterized their strategy as one that sought not to “inflame the issue.”

“The idea that they — the governors of New Jersey and New York — they’re going to let a single mother of four control the future is also crazy,” he added, referring to Ms. Kelly.

A spokesman for Mr. Cuomo flatly denied that he had any involvement. “The governor did not have any role — direct or indirect — in any press statements regarding the purported traffic study,” he said.

The case will hinge on the government’s star witness, Mr. Wildstein, a former political blogger hired to a position created specially for him at the Port Authority, who has admitted, in a guilty plea as part of a deal to cooperate with the government, that he conceived the idea to close the lanes.

Defense lawyers characterized him as crazy, a liar described even by witnesses for the prosecution as “a vicious guy,” “maniacal” and “a horrible person.”

And it was Mr. Christie, they said, who installed Mr. Wildstein at the agency to be his enforcer. Mr. Baldassare, Mr. Baroni’s lawyer, said the governor referred to Mr. Wildstein as his “fixer,” or “Mr. Wolf,” after the Harvey Keitel character in the movie “Pulp Fiction,” the guy who cleans up the dead bodies.

And everyone — Mr. Baroni included — feared crossing him.

“At the Port Authority at the time, when David Wildstein spoke, Governor Christie’s voice came out and everybody knew it,” Mr. Baldassare said. “It wasn’t just Bill. David Wildstein, based on this evidence, looks like a ventriloquist’s doll sitting on Christopher J. Christie’s lap.”

Mr. Christie, defense lawyers said, was United States attorney when he began talking to Mr. Wildstein, then the author of an anonymous and gossipy political blog in New Jersey; the governor hired him after seeing his ability to work in the shadows as a virtue.

Mr. Wildstein was a political operative so wily, Mr. Baldassare said, that he once stole Senator Frank R. Lautenberg’s jacket before a debate so that Mr. Lautenberg would have to borrow someone else’s jacket and be uncomfortable during a debate with Mr. Wildstein’s candidate, Representative Millicent H. Fenwick.

At the Port Authority, Mr. Baldassare said, he operated on “one constituent rule,” wanting to please only Mr. Christie. He and the governor played a “red light, green light” game, Mr. Critchley said, going down a list and firing people they did not like, greenlighting those who remained in their favor.

Mr. Wildstein’s goal, Mr. Critchley said, was to run Mr. Christie’s presidential campaign in one of the important early states — Iowa, New Hampshire or South Carolina. As the bridge scandal threatened to take down those presidential hopes, Mr. Critchley said, Mr. Christie pinned the blame on Ms. Kelly.

“This case is not only about traffic, it’s about a presidential campaign for the United States of America,” he said. “What she knew could be fatal to an embryonic presidential campaign.”

She was scapegoated, he argued, by a “coterie of cowards.”

“Cowards who were addicted to power,” he said. “Cowards who despite all their titles, when it became time to speak the truth, they showed they were nothing more than opportunists, ambitious opportunists.”

Patrick McGeehan contributed reporting.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: U.S. Asserts Christie Knew of Bridge Plot. Order Reprints | Today’s Paper | Subscribe